Because organic solar cells do not require frequent use of high-vacuum or high-temperature processing when fabricating them and, and low-cost raw materials can be used, there is an interest in organic solar cells as next-generation low-cost solar cells. There is particularly high interest in the Grätzel-type dye-sensitized solar cell.
The Grätzel-type dye-sensitized solar cell (see Non-Patent Citation 1) has a structure that includes a photoelectric conversion electrode in which a dye is adsorbed on a metal nanoparticle layer of titanium oxide or the like that is sintered on a transparent conductive substrate, a counter electrode formed of a conductive substrate that has a thin film of Pt or a carbon material formed thereon, and an electrolyte layer including a redox couple such as iodine that is sandwiched between these electrodes. The photoelectric conversion efficiency of this dye-sensitized solar cell greatly depends on the solar absorption capability of the dye. Although the reported value for the Grätzel-type dye-sensitized solar cell is 11% (1-cm square in size), which is the highest reported photoelectric conversion efficiency among the organic solar cells, the photoelectric conversion efficiency needs to be further enhanced in order to achieve practical use. Up to now, there has been research and development of long-wavelength absorbing dyes, and a representative dye is N719 dye, or Black Dye, which is one of the Ru metal polypyridine complexes. Even with the Black Dye used in the dye-sensitized solar cell exhibiting the highest conversion efficiency at the present time, the absorption edge is at about 900 nm, and it is necessary to shift the absorption edge further toward longer wavelengths. Under such circumstances, a complex that includes Os metal and a pyridine derivative, serving as a ligand, and that allows for a greater long-wavelength shift than the Black Dye has been investigated. When a pyridine derivative that serves as a ligand is appropriately selected, the absorption-edge wavelength of an OS dye shifts. With a dye-sensitized solar cell fabricated by using a dye in which substituents (X) of a ligand are assumed to be H, COOH, and C(CH3)3, its IPCE (Incident Photon-to-current Conversion Efficiency) starts to increase near 1100 nm and reaches about 30 to 50% at 900 nm; however, at 800 nm, the IPCE is about 50% at most (see Non-Patent Citation 2). In order to achieve high photoelectric conversion efficiency, it is essential to achieve high IPCE values in all wavelength regions, including the near-infrared region, the visible light region, and the ultraviolet region.
In addition, in the past, there has been various research on Ru complexes, including their absorption characteristics and so forth, and, although research into the absorption characteristics of, for example, Ru complexes having terpyridine and a phosphine derivative as ligands has also been carried out, absorption was not exhibited over the entire visible light region (for example, Non-Patent Citation 3).
In addition, Patent Citation 1 discloses a metal complex in which coordinate bonds are formed involving two terpyridine molecules having phosphonic acid or carboxylic acid (complex given by Expression (8) in Patent Citation 1), a metal complex in which coordinate bonds are formed involving one terpyridine molecule having phosphonic acid and a predetermined bidentate or tridentate aza ligand (complex given by Expression (9) in Patent Citation 1), and a metal complex in which coordinate bonds are formed involving bipyridine having phosphonic acid, a predetermined bidentate or tridentate aza ligand, and a predetermined monodentate ligand (complex given by Expression (10) in Patent Citation 1). However, there is no description of a metal complex in which coordinate bonds are formed involving terpyridine having phosphonic acid or carboxylic acid and a phosphorus-based ligand.